Physoderma Brown Spot, Stalk Breakage and Rot, Northern Corn Leaf Blight, and Tar Spot​

During and after harvest is a key time to evaluate a farming operation. What went well? What needs to be improved? How did my products perform? Why did someth​ing perform well or underperform?

This can also be a good time to reflect on specific agronomic challenges like corn diseases or refresh knowledge. I am going to highlight several corn diseases: Physoderma brown spot (PBS) and Physoderma stalk breakage and rot, northern corn leaf blight (NCLB), and tar spot (TS).

Physoderma brown spot in corn is a fungal pathogen caused by Physoderma maydis and is a minor disease overall. Physoderma is responsible for two possible issues: leaf blight, and stalk breakage and/or rot. The leaf blight phase of PBS rarely affects yield because the lesions generally do not consume enough leaf tissue. Lesions are typically small and round to oblong in shape (Figure 1). 

Figure 1. Physoderma brown spot.

They can vary in color from yellow to brown and as the disease progresses, lesions can coalesce and darken to a reddish brown/purple color. Look for lesions on leaves, leaf midribs, leaf sheaths and husks which typically appear prior to tasseling. Purple spots can characteristically be found along the midrib with PBS.

If PBS is found in the leaf blight phase, it does not mean that the stalk breakage and rot will follow. Also, the foliar symptoms may not be prevalent in fields with Physoderma stalk breakage and rot. Physoderma stalk breakage generally occurs at the first or second stalk nodes. When a node breaks due to Physoderma, it can appear black and the pith may be rotted.

Physoderma is a fungal pathogen that favors abundant rainfall, temperatures between 73° F to 90° F, and overwinters in and on corn residue. Tips for management include residue management, rotation and Channel® brand corn products with tolerance to PBS. Most products have an adequate tolerance to PBS and products are being evaluated for the stalk breakage and rot.

​Northern corn leaf blight is considered a major foliar disease in corn and has the potential to impact profits. Lesions are typically described as elliptical or “cigar-shaped,” 1 to 6 inches in length and gray to tan in color (Figure 2). As lesions mature, they typically darken in color and dark fungal sporulation develops within. These lesions can be found at any time but typically are most abundant after pollination. When severe infections occur, lesions can take over and destroy an entire leaf. Lesions on plants with resistance genes can appear as long, chlorotic streaks.

Figure 2. Northern corn leaf blight lesion.

Northern corn leaf blight is caused by the fungus Exserohilum turcicum and overwinters in and on corn residue. Infection requires water present on the leaf surface for six to 18 hours, moderate temperatures and high humidity. Best management practices involve selecting a Channel corn product with good NCLB resistance and high yield potential. Other practices include residue management; crop rotation; balanced soil fertility; and minimizing plant stress from weeds, insects, and other diseases. Fungicides are effective against NCLB but depending on a product’s resistance ratings, disease pressure and future weather, an application may or may not provide economic returns.

Tar spot of corn is a newer foliar corn disease in several states throughout the Midwest. In the United States, it’s a fungal pathogen caused by Phyllachora maydis and when found alone, is not known to cause economic yield loss. In Latin America, tar spot of corn results from a synergistic interaction of three fungal species: Phyllachora maydis, Monographella maydis and the hypeparasite Coniothyrium phyllachorae. Crop losses are reported in Mexico but only when infections include the second fungus, Monographella maydis (along with Phyllachora maydis ) – which this combination is not present in the United States.

Tar spot of corn appears to favor cool, humid conditions, and lesions are distinctive and appear as if there are spots of tar on an infected leaf (Figure 3). Symptoms typically begin as oval to irregularly shaped bleached to brown lesions where black spore-producing structures eventually form. If you run a finger over the leaf, it can feel bumpy from the protruding lesions. Symptoms of tar spot on corn can also be found on leaf sheaths and husks. There is still more to be learned about this corn disease in the United States. So far, we also know that it’s not seedborne, only infects corn, and spreads with fresh or dried corn leaves and husks. 

Figure 3. Tar spot with gray leaf spot lesions.

Being able to identify corn foliar diseases can help when determining a solution for the current and following years. In my experience, many successful farmers keep detailed information for each field on inputs, product placement, planting dates, observations, disease history and harvest data. They understand the importance of reviewing genetics, traits, fertility and pest control coupled with crop scouting to proactively address potential challenges and keeping up on the latest agronomic information. It can be important to partner with trusted advisors so feel free to reach out to your local Channel Seedsman with any questions.​​​​​​​​​​​

Sources:

Robertson, A., Mueller, D., Salaau-Rojas, E., and Munkvold, G. 2013. Stalk breakage and rot caused by Physoderma in Iowa. https://crops.extension.iastate.edu/.%20cropnews/2013/09/stalk-breakage-and-rot-caused-physoderma-iowa​

Mahuku, G., Shrestha, R., and San Vicente, F. 2013. Tar spot complex of maize: facts and actions. CIMMYT. https://www.researchgate.net

Additional Information:

Identifying Physoderma brown spot. Agronomy ADVICE. Channel.com

Identification and management of northern corn leaf blight. Agronomy ADVICE. Channel.com

Tar spot in corn. Agronomy ALERT. Channel.com

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